/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
        TreeNode* root;

        if(root1 == nullptr && root2 == nullptr){
            root = nullptr;
        } if(root1 != nullptr && root2 != nullptr){
            root = root1;
            root1->val = root1->val + root2->val;
            root1->left = mergeTrees(root1->left, root2->left);
            root1->right = mergeTrees(root1->right, root2->right);
        } else if(root1 != nullptr){
            root = root1;
            root1->left = mergeTrees(root1->left, nullptr);
            root1->right = mergeTrees(root1->right, nullptr);
        } else if(root2 != nullptr){
            root = root2;
            root2->left = mergeTrees(nullptr, root2->left);
            root2->right = mergeTrees(nullptr, root2->right);
        }

        return root;
    }
};